1. Field of the Invention
The invention relates to radar systems generally and, more specifically, to moving target indicator subsystems therefor.
2. Description of the Prior Art
MTI (moving target indicator) radar has been intensively developed and exists in various forms, both analog and digital. The technical and patent literature extensively describes MTI and its variations. For example, the text, RADAR HANDBOOK, by Merrill I. Skolnik (McGraw-Hill Book Company, 1970) devotes chapter 17 to the subject. FIG. 4 of that chapter of the aforementioned text describes a typical prior art pulse radar MTI system. A pulsed microwave transmitter and receiver operating through a duplexer and common antenna system is standard, as is some means for "remembering" the transmitted RF phase. The frequency and phase of an IF domain coherent oscillator (coho) may be reset at each transmitted pulse and held throughout the pulse repetition period in such prior art systems. Return echo signals reduced to the IF frequency domain are phase detected against the coho, and the result is what is commonly referred to as bipolar video. Actually, these video signals have an amplitude and polarity in each range bin which is the result of vector addition against the coho in each pulse repetition period. Signals corresponding to moving targets assume polarities and amplitudes ranging between a maximum positive and a maximum negative value as a result of this phase detection over a number of succesive pulse repetition periods.
Fixed targets tend to assume invariant amplitude and polarity characteristics and, thus, are easily cancelled by subtraction of one signal train corresponding to a given pulse repetition period from an adjacent pulse repetition period on a continuous basis. For this, long delay line type cancellers are sometimes used, although digital memories and processors are also sometimes employed therefor. Signals corresponding to moving targets, because of their variation of amplitude and polarity among pulse repetition periods, are not cancelled; hence the term, moving target indicator.
Upon close examination of such prior art systems, it will be realized that the frequency of the coho cannot reflect the variations of transmitted energy phase history throughout the duration of each transmitted pulse. This transmitted pulse duration basically constitutes the range resolution cell of a pulsed radar system, the term "range bin" being applied thereto.
Normally, it is assumed that the phase-lock mechanism, employed for locking-in the coho during each transmitted pulse, chooses a compromise steady state value sometime during the transmitted pulse duration. Quite obviously, it would be advantageous, however, from the point of view of quality of cancellation if, during each range bin, the phase foibles of a noncoherent transmitter (such as a magnetron, etc.) could be faithfully reproduced in each subsequent range bin during the pulse repetition period (receiving time).
The manner in which the present invention provides a coherent reference generator, replacing the prior art coho device and providing a more faithful reproduction of the phase history throughout a transmitted pulse, will be evident as this description proceeds.